1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device using a thin film transistor (TFT) formed on a transparent substrate such as glass or plastics, and a driving method of the same. In addition, the invention relates to electronic apparatuses using the display device.
2. Description of the Related Art
In recent years, with the advance of the communication technology, mobile phones have been widely used. In future, transmission of moving images and a larger volume of information are expected. On the other hand, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information terminals called PDA originated in electronic notebooks have also been produced in large quantities and widely used. In addition, with the development of display devices, the majority of portable information devices are equipped with a flat panel display.
Moreover, according to recent techniques, an active matrix display device tends to be used as a display device for the above portable information device and the like. In the active matrix display device, a TFT is arranged in each pixel and a display screen is controlled by the TFT. Compared to a passive matrix display device, such an active matrix display device has advantages in that it achieves high performance and high image quality, and can handle moving images. Therefore, it is expected that the mainstream of liquid crystal display devices will also shift from passive matrix types to active matrix types.
An active matrix display device has been manufactured using a polycrystalline semiconductor such as polysilicon formed at a temperature of 500° C. or less at highest, which is rather low as compared to the conventional condition of 1000° C. or more. Such an active matrix display device has advantages in that, in addition to a pixel, a driver circuit can be integrally formed around a pixel portion, which makes it possible to realize downsizing and high definition of a display device. Thus, such a display device is expected to be more widely used in future.
A description is given below on the operation of a pixel portion in an active matrix liquid crystal display device. FIG. 2 shows a configuration example of an active matrix liquid crystal display device. One pixel 220 comprises a source signal line 203, a gate signal line 205, a capacitor line 219, a pixel TFT 207, a storage capacitor 211, and a liquid crystal 215. However, the capacitor line may not necessarily be provided if other wiring and the like can concurrently serve as the capacitor line. A gate electrode of the pixel TFT 207 is connected to the gate signal line 205. Further, either of a drain region or a source region of the pixel TFT 207 is connected to the source signal line 203 whereas the other is connected to the storage capacitor 211 and the liquid crystal 215.
The gate signal line 205 and a gate signal line 206 are selected sequentially in accordance with a line cycle. Provided that the pixel TFT 207 and a pixel TFT 209 are N-channel types, the gate signal line 205 becomes active when it is Hi, thus the pixel TFTs 207 and 209 are turned ON. As the pixel TFTs 207 and 209 are turned ON, potentials of the source signal line 203 and a source signal line 204 are respectively written into the storage capacitor 211 and a storage capacitor 213 and into the liquid crystal 215 and a liquid crystal 217. In the next line period, the adjacent gate signal line 206 becomes active and potentials of pixel TFTs 208 and 210 become Hi. Then, the potentials of the source signal lines 203 and 204 are respectively written into storage capacitors 212 and 214 and into liquid crystals 216 and 218 in a similar fashion. The liquid crystals 215 to 218 are aligned in accordance with the written potentials and change light transmissivity. In this manner, the active matrix liquid crystal display performs a display operation using a liquid crystal as a light shutter.
Also, as shown in FIG. 14, an active matrix display device which performs a display operation with a static RAM (SRAM) inside its pixel portion and is also developed. (See Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-open No. Hei8-286170
In FIG. 14, one pixel 1407 comprises an SRAM 1403, switches 1405 and 1406, and a liquid crystal 1404. A source signal line driver circuit 1401 outputs an video signal to source signal lines 1408 and 1409. When a gate signal line 1410 is selected by a gate signal line driver circuit 1402, the video signal is written into the SRAM 1403 via the source signal lines 1408 and 1409. Either of the switches 1405 and 1406 operates based on the stored signal in the SRAM 1403 and either a potential Va or Vb is applied to the liquid crystal 1404. This signal is maintained until the next video signal is written into the SRAM 1403. In this manner, a display operation is performed.